WATER A PRECIOUS RESOURCE 7th CLASS
INTRODUCTION
We all depend on the same vital substance water, diverse by its nature, it is solid, vapour and liquid. It is in the air, on the earth surface and within the ground ever-changing and giving shape to a dramatic range of natural ecosystem.
For the earth’s inhabitants, diversity of the resource also means great disparities in well being and development. As we degrade the quality of our water and modify the natural ecosystems on which people and life depend, we also threaten our own survival.
AVAILABILITY OF WATER
Water is the most widely occurring substance on this planet. Water has shaped the earth’s evolution and continues to fasion its programmes. This is the water preCipitated from the atmosphere on to land, where it may be stored in liquid or solid form, and can move laterally and vertically and between one phase and another phase by evaporation, condensation, freezing and thawing on the land surface. This water can travel at widely differing velocities usually by predictable pathway's which can slowly change with time. We are aware that about 71 % of the earth's surface is covered with water. Almost all the water on the earth is contained in the seas and oceans, rivers, lakes , ice caps, as groundwater and in the atmosphere. However, most of this water is not fit for human consumption.
CMPOSITION OF WATERO
Water is a compound. Its constituent elements are hydrogen and oxygen. Water is a stable compound, but can be broken into hydrogen and oxygen when electric current is passed through it. Its chemical formula is H2O.
PHYSICAL PROPERTIES OF WATER
The properties of water are different from those of oxygen and hydrogen. For example, water extinguishes fire, but oxygen supports combustion and hydrogen burns with pop sound.
1. Pure water is a colourless, odourless, tasteless, transparent liquid.
2. Water exists in all the three stales, solid (as ice), liquid (as water) and gas (as water vapour).
3. Pure water boils at 100°C.
4. Pure water freezes at 0°C.
FORMS OF WATER
When water circulates through the water cycle it can be found in all the three forms i.e., solid, liquid and gas· at any given time somewhere on the earth. The solid form, snow and ice, is present as ice caps at he poles of the earth, snow covered mountains and glaciers. Liquid water is present in oceans, lakes, rivers and even underground. The gaseous form is the water vapour present in the air around us, The continuous cycling of water among its three forms keeps the total amount of water on the earth constant.
WATER CYCLE
The supply of water in nature does not run out. This is because water is continuously recycled in the water cycle.
1. Heat from the sun causes the water on the earth's surface to evaporate. The vapour rises, cools and condenses to form tiny water droplets. These droplets form clouds.
2. The clouds get carried along by air currents. They cool and the droplets join to form larger drops. These fall as mist.
3. If the temperature in the region is very low, these water droplets fall as hail, sleet or snow,
4. Some of the rain water flows along the ground as streams. Some soaks through the ground and then reappears as springs, Streams and springs join to form rivers. Rivers flow back into the sea. Thus the water cycle is complete.